A laser beam generates a monochromatic ray of light, which features only one single wavelength (from Greek 'mono chromos' = one colour). The benefits of this method, which allows the highly selective detection even of minimum gas concentrations using an inexpensive technology, provide reasonable grounds to apply photoacoustic detection in practical situations.

Acoustics and ripe fruits

Tips and tricks that Grandma used to know advise us to "always keep apples or bananas away from other fruits to prevent them from rotting early." And Grandma was right. When maturing, fruits release gases the amount and composition of which may differ depending on the specific kind of fruit, with ethylene playing a crucial role in most cases. Their specific emission behaviour is the reason why fruits that are stored close to apples tend to ripen faster, but also to decay faster. The question how fruit can be prevented from deteriorating during long-distance transport, while avoiding unripe delivery to the supermarkets, is a central research topic investigated by Dr. rer. nat. Judit Angster in collaboration with a scientific team of Campos University in Brazil (UENF). The scientists apply findings and models from the field of physics to explore the perfect degree of ripeness of fruits. As the enthusiastic physicist from Fraunhofer Institute for Building Physics IBP declares, "it is not just about ensuring the optimum fruit taste for consumers - we are also looking to find a way to reduce the tremendous crop losses, which are currently between 50 and 70 percent."

Consumers are becoming increasingly demanding: No matter if they prefer pineapples, bananas or kiwi fruit – not only lovers of exotic fruits want to enjoy their favourite fruit when its aroma is most intense. But to ensure the perfect, delightful taste there is more to it than meets the eye. Ideally, sweetness and juiciness will please the palate and delight the consumer with fresh, fully ripe fruitiness. All over the world, stringent regulations apply to production, storage, processing, and preparation of foodstuffs to protect consumers. As a consequence, quality controls during transportation are becoming increasingly important. Now this is where fruit logistics come into play. Due to defined and controlled processes, fruit logistics ensure that fruits will be delivered to the customers at premium quality. "If we focus on this interface, we will be able to improve quality and thus meet or even surpass the high standards of the consumers," Judit Angster is convinced. Her plan is to apply physics to make sure that fruits will be perfectly ripe.

As temperature and thermal conditions have an influence on the fruits it seems logical that they need continuous monitoring during transportation. Yet there are a few other parameters that need to be considered. Regarding fruit logistics, the key factor is ethylene. Fruits of all kinds emit ethylene during their entire life cycles. Ethylene is a gaseous, colourless hormone with a faint, sweet odour which is released as a metabolic product by plants and accelerates the natural ripening process. Studies have shown that there is a direct correlation between the degree of ripeness and the ethylene concentration released by fruits. Judit Angster has already intensively dealt with all these facts: "We apply these findings to develop measuring systems, which are used for monitoring the ethylene content during transport," the physicist explains. "State-of-the-art technology, as it is used in photoacoustic applications, can meet these requirements. This technology allows influencing and assessing the quality of fruit more efficiently."

But what does the principle of photoacoustics really mean and how can the exact ethylene content of fruits be measured? "We apply physical laws," Angster characterizes her research activities. "During a photoacoustic measuring process, we introduce a mixture of gases into an emission test chamber. High-performance lasers then direct pulsed light onto the test configuration. Due to the pulsed operation of the laser source, periodic fluctuations of temperature and pressure will occur inside the measuring chamber, which are recorded by a microphone previously installed in the measuring chamber. The proportionality of sound power level, light energy and the absorption coefficient of the gas provide information on the concentration level of the particular molecule – ethylene, in this case – in the gas mixture." So if one wants to determine the exact measure of the ethylene concentration, it can be derived from the power of the acoustic signals. This data enables acousticians to assess the degree of the fruit's ripeness while establishing and monitoring the optimum storage environment inside the container. "The facts that fruits have different requirements regarding the environmental atmosphere and that nature exerts its influence in the containers, too (by triggering ripening processes, for instance) need to be followed very closely, of course," Judit Angster adds.

So it's all about energy interchange processes, actually about the conversion of light energy to kinetic energy. The term is derived from the Greek word "kinesis" = "movement" and defines the energy an object contains due to its movement. A light beam that consists of a single wavelength only is called monochromatic (from Greek 'mono chromos' = one colour). This kind of light does not occur naturally and can only be obtained artificially via a laser beam. Regarding the application of gas detection in photoacoustics, the wavelength of the monochromatic light is crucial. The measuring system adapts the wavelength to the absorption line of the specific target molecule that is to be detected. But what are absorption lines, actually? Every chemical element leaves a characteristic "finger print" in the form of dark lines in the colour spectrum, similar to a barcode. The pattern of these lines allows experts to identify the elements the material the light has passed through is composed of. Joseph von Fraunhofer, the patron to give his name to the Fraunhofer-Gesellschaft, discovered this relationship in 1813.

The advantages of this method are convincing. Using the cost-efficient technology of photoacoustics, even minimum gas concentrations can be precisely detected - good reasons to implement this method in practical applications. Yet the team around Dr. Judit Angster has many more ideas concerning further areas in which this technology offers promising and interesting prospects. They see major opportunities for the use of photoacoustic gas sensors in environmental protection, medical diagnostics, with regard to biological issues as well as in the fields of security or quality assurance. Angster has committed herself to establish Photoacoustics at the Fraunhofer IBP as a new area of research and development. The internationally acclaimed Institut für Physikalische Chemie [Institute of Physical Chemistry] (Prof. Peter Hess) of Heidelberg University, whose activities are continued at Fraunhofer IBP with a stronger emphasis on practical applications, is also part of the background. The renowned Heidelberg expert for gas-phase photoacoustics, Prof. András Miklós, has become integrated in this new field of research at Fraunhofer IBP.

RESEARCH IN FOCUS

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